Well cleaning apparatus

ABSTRACT

An apparatus for cleaning calcites and other extraneous matter from the perforated casing of a sub-terrainian water well comprises a plurality of elongate explosive members  14  for lowering into the well, detonators  16  for detonating each member  14  to produce an explosion which propagates longitudinally of the well, and a plurality of short tubular attenuators  19  circumscribing the explosive members  14  at intervals along the length thereof. Each tubular attenuator  19  comprises a steel tube  20  and an annular ring  21  extending around the tube at a point along the length thereof.  
     The tensile strength of the tube  20  is arranged to be less than that of the force produced by the explosion, whereas the tensile strength of the ring  21  is arranged to be greater than that of the force produced by the explosion.  
     The rings  21  thus serve to attenuate the pressure wave substantially more than tubes  20 , thereby helping to produce a more pronounced attenuation. Furthermore, the shape and direction of the pressure wave of the explosion can be altered according to the relative positioning of the ring  21  on the tubes  20.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to an apparatus for cleaning sub-terrainian water wells.

[0003] 2. Related Background Art

[0004] Sub-terrainian water wells generally comprise a bore hole which is sunk deep into the ground and lined with a perforated tube or so-called casing, through which water from the surrounding rock can permeate.

[0005] Over time, the perforations become clogged with extraneous matter such as calcites.

[0006] In order to overcome this problem, it has been proposed to clean the casing of wells using an explosive charge lowered into the well which, when detonated, dislodges a substantial amount of the debris and other extraneous matter clogging the perforations.

[0007] U.S. Pat. No. 3,721,297 discloses one such well cleaning apparatus comprising a plurality of explosive charges arranged longitudinally of a cable lowered down the well. In use, the charges are successively detonated at time intervals to produce a pulsed pressure wave which travels longitudinally along the length of the well: this pulsing has been found to be more effective at cleaning wells than a single charge.

[0008] A disadvantage of this arrangement is that government regulations prohibit the transportation of explosive goods carrying detonators. Thus, in order to overcome this problem, the detonators have been attached to the explosive on site: this is both difficult and time consuming, since there are often at least fifteen different separate explosive charges.

[0009] U.S. Pat. No. 4,757,863 discloses a solution to this problem in the form of an elongate explosive charge having a plurality of steel sleeves placed around the charge at intervals along its length. In use, as the charge deflagrates from one end, an outwardly-directed pressure wave travels longitudinally along the well, and the sleeves, by attenuating the pressure wave at positions along the length of the charge, in effect provide a sufficient micro delay in the propagation of the pressure along the well.

[0010] A disadvantage of this arrangement, is that the attenuation of the explosion caused by the sleeves is small and as such the desired pulsation is not always achieved. Furthermore, the shape of the pulse that is produced is not particularly beneficial to the cleaning of the well.

[0011] We have now devised a well cleaning apparatus which alleviates the above-mentioned problems.

SUMMARY OF THE INVENTION

[0012] In accordance with this invention, there is provided a well cleaning apparatus comprising an elongate explosive member for mounting longitudinally in a well, means for detonating said member to produce an explosion which propagates longitudinally thereof, and a plurality of short tubular attenuators circumscribing the explosive member at intervals along the length thereof, each tubular attenuator having a first and second circumferentially extending zones, arranged longitudinally with respect to the explosive member, the first zone having a tensile strength which is substantially greater than that of the second zone.

[0013] The first zone serves to attenuate the pressure wave substantially more than the second zone does and thus helps to produce a more pronounced attenuation. The shape and direction of the pressure wave can thus be altered according to the relative positioning and strengths of the two zones on each attenuator.

[0014] We have found that the tubular steel attenuators used in U.S. Pat. No. 4,757,863 are blown apart by the force of the explosion. Thus, the first zone preferably has a tensile strength which is greater than the force produced by the explosive member and thus fully attenuates the pressure wave.

[0015] Preferably the tensile strength of the second zone is less than the force produced by the explosion and as such this region of the attenuator partially attenuates the pressure wave and is then blown apart by the explosion, producing a sudden and strong pressure wave, the direction of which can be varied according to the position of the first zone.

[0016] Preferably, the second zone is provided at the axial end of the attenuator facing against the direction in which the explosion propagates. As the explosion reaches the attenuator, the second region, at its proximal end, attenuates the pressure wave to a degree and is then blown apart. The fragments of the second region remain attached to the first (stronger) region and these fragments flare out and act as a baffle to direct the pressure wave outwardly and rearwardly back towards the duration of propagation of the explosion.

[0017] Additionally and/or alternatively, the first zone preferably extends around the axial end of the attenuator which faces in the direction in which the explosion propagates: this serves to direct the explosion outwardly and forwardly in the direction of propagation of the explosion.

[0018] Preferably each attenuator comprises a metal tube and an annular ring extending around the tube to increase the wall thickness and hence the strength of the attenuator at that point.

[0019] Preferably the wall of the ring is circular in cross-section.

[0020] Preferably the tubes are formed of carbon steel and preferably comprise a wall thickness of substantially 1.65 mm.

[0021] Preferably a detonator is disposed at one end of the explosive member, a tube extending around said end of the explosive member to shield all of the radially directed pressure at the detonator, thus making the apparatus safer for use in certain types of well where the well casing has shown slight damage adjacent the detonator.

[0022] Preferably the distal end of the apparatus comprises a circular-section collection device arranged to be a close fit within the casing. In use, the collection device is arranged below the explosive member in the well and serves to catch matter dislodged from the well casing and debris from the explosive member.

[0023] Preferably the collection device is perforated, so that it is able to travel longitudinally of the well through the water therein.

[0024] Preferably the explosive member comprises a plurality of separate sections arranged longitudinally of each other, each section comprising a detonator which can be programmed to ignite at a predetermined variable time after a common detonation signal is applied to the apparatus. In this manner sequential triggering of several sections can be achieved, and variable delays can be programmed into the detonators on site, in order to precisely control the timing and sequence of the explosion.

[0025] Preferably the explosive strength of the member increases towards one end thereof, in order to compensate for the increase in water pressure towards the bottom of the well.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Embodiments of this invention will now be described by way of examples only and with reference to the accompanying drawings, in which:

[0027]FIG. 1 is a longitudinal sectional view of a well casing having an apparatus in accordance with this invention installed therein;

[0028]FIG. 2 is a side view of a portion of the apparatus of FIG. 1; and

[0029]FIG. 3 is a sectional view along the line III-III of FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Referring to FIG. 1 of the drawings, there is shown a bore hole 10 formed vertically in the ground to form a well into which water from the surrounding rock flows. The well is lined with a perforated tubular steel casing 11.

[0031] In order to clean debris and other extraneous matter clogging the perforations, a well cleaning apparatus 13 is lowered into the well.

[0032] The apparatus 13 is connected to a winch via an elongate steel cable 12. The apparatus 13 comprises a plurality of elongate explosive members 14 mounted longitudinally of a second cable 15 that is attached to the lower end of the cable 12.

[0033] The lower end of the cable 15 is attached to a sample basket 16 which is arranged to catch samples of the matter dislodged from the wall of the casing 11. A larger diameter open-wire basket is mounted below the sample basket 16 to clean the well of debris dislodged from the casing 11 and of remnants of the explosive members 14.

[0034] Referring to FIG. 2 of the drawings, the apparatus comprises three separate elongate explosive members 14 mounted co-axially of each other and longitudinally of the cable 15. The upper and lower members 14 each have a detonator 16 mounted at their respective upper ends whilst the central member 14 has a detonator 16 mounted at its lower end. The detonators 16 are interconnected by a control wire 17 which extends to a detonating device at the well surface. Each detonator 16 is contained within a perforated sleeve 18.

[0035] The explosive charge contained within the three explosive members 14 successively increases from the upper member to the lower member 14 over a distance which can be up to 100 meters.

[0036] Each explosive member 14 comprises a plurality of tubular attenuator devices 19 which circumvent the explosive member at intervals along its length. The spacing between each attenuator 19 is preferably short compared with the overall length of the member 14 and typically may be as little as 1 meter.

[0037] Referring to FIG. 3 of the drawings, each attenuator comprises a carbon steel tube 20 of uniform wall thickness. An annular ring 21, also of carbon steel, extends around the tube 20 intermediate its opposite ends. However, the position of the ring 21 could, if necessary, be positioned elsewhere on the tube to modify the shape of the pressure wave.

[0038] In use, when a detonation signal is applied to the wire 17 the detonators 16 ignite the explosive members 14 at timed intervals pre-programmed into the detonators. The resultant explosions cause respective pressure waves which propagate along the members from their detonator end towards their opposite end.

[0039] Upon reaching an attenuator 19, the pressure wave is initially contained by the tube 20. However, the force produced by the pressure wave is greater than the tensile strength of the tube material and thus the tube eventually blows apart and releases a sudden and intense pressure wave.

[0040] The combined wall thickness of the tube 20 and ring 21 is sufficient to withstand the pressure and remains intact. The ring 21 effectively forms a waistband around the tube 20 to stop it fragmenting completely and we have found that what happens is that the proximal end of the attenuator flares outwardly in the direction of the propagation of the explosion, as shown in outline, thereby directing the pressure wave that its released both longitudinally and radially outwardly of the casing: this combined longitudinal and radial pressure wave is more effective at dislodging debris on the wall of the casing 11 than the conventional method solely involving a radial pressure wave. Moreover, the action of the attenuators can be used to steer or concentrate the pressure wave at desired areas of the casing 11.

[0041] The explosion continues to propagate past the ring 21, whereupon the distal end of the attenuator suddenly blows apart to release a further intense pressure wave in the opposite direction.

[0042] The attenuators are mounted relatively close together and thus a rapidly moving and pulsating pressure wave is produced which is extremely effective at clearing debris from the casing 11 without causing any damage.

[0043] Following clearing, the cable 12 is re-wound and the wire basket 17 scours the well 10 for any debris and/or remnants of the explosive members 14.

[0044] In an alternative embodiment, a different number of explosive members may be used, for example two or four mounted longitudinally of each other and perhaps spaced apart. The position of the detonators may also be varied so that the explosive members burn upwards or downwards depending on the circumstances.

[0045] While the preferred embodiment of the invention has been shown and described, it will be understood by those skilled in the art that changes of modifications may be made thereto without departing from the true spirit and scope of the invention. 

We claim:
 1. A well cleaning apparatus comprising an elongate explosive member for mounting longitudinally in a well, means for detonating said member to produce an explosion which propagates longitudinally thereof, and a plurality of short tubular attenuators circumscribing the explosive member at intervals along the length thereof, each tubular attenuator having a first and second circumferentially extending zones, arranged longitudinally with respect to the explosive member, the first zone having a tensile strength which is substantially greater than that of the second zone.
 2. A well cleaning apparatus as claimed in claim 1, in which the first zone has a tensile strength which is greater than the force produced by the explosion.
 3. A well cleaning apparatus as claimed in claim 1, in which the tensile strength of the second zone is less than the force produced by the explosion.
 4. A well cleaning apparatus as claimed in claim 1, in which the second zone is provided at the axial end of the attenuator facing against the direction in which the explosion propagates.
 5. A well cleaning apparatus as claimed in claim 1, in which the first zone preferably extends around the axial end of the attenuator which faces in the direction in which the explosion propagates.
 6. A well cleaning apparatus as claimed in claim 1, in which each attenuator comprises a metal tube and an annular ring extending around the tube.
 7. A well cleaning apparatus as claimed in claim 6, in which the wall of the ring is circular in cross-section.
 8. A well cleaning apparatus as claimed in claim 6, in which the tubes are formed of carbon steel.
 9. A well cleaning apparatus as claimed in claim 6, in which the tubes comprise a wall thickness of substantially 1.65 mm.
 10. A well cleaning apparatus as claimed in claim 1, comprising a detonator is disposed at one end of the explosive member.
 11. A well cleaning apparatus as claimed in claim 1, in which the distal end of the apparatus comprises a circular-section collection device arranged to be a close fit within the well casing.
 12. A well cleaning apparatus as claimed in claim 11, in which the collection device is perforated.
 13. A well cleaning apparatus as claimed in claim 11, in which the explosive member comprises a plurality of separate sections arranged longitudinally of each other, each section comprising a detonator which can be programmed to ignite at a predetermined variable time after a common detonation signal is applied to the apparatus.
 14. A well cleaning apparatus as claimed in claim 11, in which the explosive strength of the elongate explosive member increases towards one end thereof. 